Thin film uniformity (i.e., substantially constant thickness of the film throughout) is an important criterion in the production of semiconductor and LCD devices, to obtain good performance and viable components over the entirety of a work piece. A susceptor is a mechanical part that holds a substrate in a processing chamber for a fabrication step, such as chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD) or physical vapor deposition (PVD), for example. The susceptor includes a substrate mounting plate mounted on a stem, along with a lift assembly for raising and lowering the substrate within the processing chamber. The substrate mounting plate is heated to facilitate the fabrication process. Typically, a heating element is disposed within the mounting plate. Most films deposited by CVD are deposited with source materials in a process chamber into which at least one of several types of energy (e.g., plasma, thermal, microwave) are inputted to facilitate the deposition process. The source materials are, of course, dependent upon the type of layer to be deposited, and may include gaseous materials such as SiH4, H2, N2, NH3, PH3, CH4, Si2H6, and O2; and/or liquid source materials which may include metal ions and organosilicate components like TEOS, for example. The films are very sensitive to temperature conditions as they are being deposited, especially those deposited with an organosilicate liquid source, since the vapor pressures of the organosilicate liquid sources are highly temperature dependent. Consequently, temperature control is a key factor in achieving film consistency when depositing thin films on large surface area substrates, such as glass plates used in the flat panel industry.
In contrast to depositions on semiconductor wafers, which occur on a relatively small scale (even the move from 200 mm to 300 mm is small in comparison with substrates used in the flat panel industry, which can run from 550 mm×650 mm and upwards of 1 m×1.2 m) depositions performed on large scale flat panel substrates have an additional set of problems to be addressed which are not critical when depositing on semiconductor wafers. A major difference is that the flat panel substrates are generally glass, which is significantly less thermally stable than a silicon wafer. Glass substrates, as opposed to silicon wafers cannot be processed above about 600° C. since too much structural stability is lost above this temperature as the glass begins to liquefy. Coupling this problem with the large surface area of the flat panels gives rise to serious concerns over bowing or sagging of the substrate in the middle portion thereof during processing at elevated temperatures. Additionally, the relatively small surface area of a semiconductor wafer facilitates the striking of a small, tightly controlled plasma for PECVD processes, whereas control and consistency of a plasma over an entire flat panel is much more challenging.
In this regard, the uniform deposition of organic components, TEOS in particular, to form organosilicate films on flat panels has proven extremely problematic, as research over the last five years has not yet been successful in providing a solution for manufacturing thin organosilicate films, using TEOS as a precursor, on flat panels of 550×650 mm or greater with ≦10% film uniformity.
As the substrate size increases, temperature control of the film deposition processes becomes even more critical because of the larger surface area and greater temperature variances across the face of the substrate, compared to what occurs with a substantially smaller substrate. Further, in examples such as flat panels, the film uniformity is a key property in determining thee performance of the product, since substantially the entire substrate may be used as the final product, as compared with an example of a semiconductor wafer, which is divided into many components and therefore each final component is only dependent upon the uniformity of the film in a much smaller area in and immediately around a portion of the wafer.
Generally, the susceptors in the prior art include a single heating element that inputs energy to the susceptor (and thus the substrate) as a whole, with feedback to vary the temperature of the substrate by varying the input through the heating element. U.S. Pat. No. 5,977,519 discloses a susceptor having dual heating elements with dual and generally parallel loops, to provide for a generally radially symmetric temperature distribution across the mounting plate while allowing for heat losses at the outside surface. However, this patent does not address the temperature dependence of the films, particularly the organosilicate films, but merely aims to compensate for heat losses at the outside surface, so as to maintain a generally even heating of the substrate.
Similarly, U.S. Pat. No. 5,844,205 discloses a substrate support structure that includes a pair of heating elements arranged in inner and outer loops so that the perimeter of the support structure may be heated to a higher temperature than the interior, for example. This control is performed to compensate for the greater heat losses that are experienced at the perimeter of the support structure. Thus, the goal of the control is to attempt to provide a uniform substrate temperature by compensating with additional heating of the substrate near the perimeter. However, similar to U.S. Pat. No. 5,977,519, this patent does not address the temperature dependence of the films, particularly organosilicate films, but merely aims to compensate for heat losses at the outside surface so as to maintain a generally even heating of the substrate.
U.S. Pat. No. 5,534,072 discloses a multi-chamber CVD processing system in which multiple lamp heaters are positioned in back of a substrate and provided with separate power controllers to vary the light supplied by each lamp heater in an effort to attain uniform temperature over the entire substrate surface. Additionally, a stepped area is machined on the susceptor surface that is in contact with the substrate. By controlling the step-machined region and its depth, the disclosure indicates that it is possible to make the temperature distribution on the substrate surface more uniform. As with the previously discussed patent, the goal of this patent is also to achieve temperature uniformity over the substrate during processing. This patent does not address the temperature dependence of the film, particularly the organosilicate films, in any way other than to generally discuss that temperature uniformity is desirable.
U.S. Pat. No. 6,225,601 discloses a technique for heating a susceptor in which the temperatures of first and second heating elements are controlled so that the difference between the temperatures of the first and second heating elements does not exceed a predetermined value while the temperatures of the heating elements are raised to their respective final temperature setpoints. Thus, this patent is directed primarily to a control system for controlling the relative temperatures between heating elements as they are heated up. This patent does not address the temperature dependence of organosilicate films, much less those films formed using TEOS.
With the trend being to move to larger and larger flat panels, improved temperature controls are needed to insure that well-performing products are achieved through film deposition processes. A need remains for a solution that will consistently produce relatively uniform thin films of organosilicate films, and particularly those formed using TEOS as a precursor, on relatively large scale substrates, such as flat panels.